While I'm not disagreeing on the quality of Flask, "x uses y, so y must be stable" is the same kind of argument as "Facebook runs on PHP, so PHP must be blazing fast." Yeah, there are large web companies using all sorts of tech, but what else they're using alongside it and how they've adapted it to their needs make this sort of commentary questionable at best.

It's definitely nice to see other responses here that are from people who are, first hand, using Flask in high-traffic live deployments. I'd just be careful about drawing conclusions based on who has it somewhere in their stack without any context.